SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    2974, 2681, 2682, 296, 281, 277, 2684, 
    2673, 620, 2672, 2683, 2473, 2679, 3020, 
    2674, 2676, 290, 2671, 2677, 2494, 2680, 
    508, 2973, 1186, 2678, 3055, 2438, 2670, 
    2463, 782, 2675, 2453, 2488
  )

Query time 0.00040

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "37.51"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": [
        "parent"
      ],
      "key": "parent",
      "used_key_parts": [
        "parent_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 65,
      "rows_produced_per_join": 65,
      "filtered": "100.00",
      "index_condition": "(`cscart_db`.`cscart_categories`.`parent_id` in (2974,2681,2682,296,281,277,2684,2673,620,2672,2683,2473,2679,3020,2674,2676,290,2671,2677,2494,2680,508,2973,1186,2678,3055,2438,2670,2463,782,2675,2453,2488))",
      "cost_info": {
        "read_cost": "31.01",
        "eval_cost": "6.50",
        "prefix_cost": "37.51",
        "data_read_per_join": "181K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ]
    }
  }
}